1. Technical Field
The present application relates to a method to control a gas engine and a gas engine system thereof, the engine being provided with: either a turbocharger or a supercharger through which air is supplied to the engine [henceforth in this application, whenever the word ‘turbocharger’ appears, it could equally well be replaced by ‘supercharger’]; a first gas control valve that controls flow-rates of fuel-gas to be supplied to each cylinder of the engine; whereby, the fuel-gas that is regulated by the first gas valve and the air that is supplied through the turbocharger are mixed so as to form a prescribed air-fuel ratio; and, the engine burns the supplied fuel-gas under conditions of the prescribed air-fuel ratio; and in particular this invention relates to a method to control a gas engine and a gas engine system thereof, capable of controlling the air-fuel ratio with a high degree of accuracy even in the case of using a fuel gas having a low calorie so as to be likely to vary its calorific value or in the case of large variation in the output power of an engine.
2. Description of the Related Art
Heretofore, of gas engines, a small-sized gas engine has in particular utilized a pre-turbocharger intake system in which fuel gas and air are mixed, in general, upstream of a turbocharger, and are then fed into a combustion chamber.
Meanwhile, in most of conventional large-sized gas engines, fuel gas has been fed into a combustion chamber by means of a fuel gas regulator valve located just before each of cylinders since a mixing ratio between fuel gas and air (that is, an air-fuel ratio) and a gas charge should be uniform for every cylinder. With the use of this system, the air-fuel ratio and the gas charge which are inputted into every cylinder can be uniform, and the working in every cylinder can become more efficient, and further, because of the configuration that the fuel gas and air are mixed just before a cylinder, an inflammable zone in a gas supply passage can be shortened, thereby it is possible to enhance the safety.
Further, Patent Document 1 (Japanese Patent Laid-Open No. 2001-132550) proposes a technology in combination of both systems as stated above. In this technology, fuel gas pressurized by a gas compressor is fed into a cylinder inlet in a intake passage or into a cylinder while fuel gas before being compressed by a gas compressor is fed into an air passage upstream of a turbocharger, and the supply of fuel gas into the cylinder side and the supply of fuel gas into the upstream side of the turbocharger can be changed over.
However, in the configuration disclosed in the Patent Document 1, it is required to compress the fuel gas up to a pressure higher than a supercharged air pressure in the fuel gas supply system in which the fuel gas compressed by the gas compressor is fed into the cylinder inlet of the intake passage or into the cylinder, but the gas compressor has to have a large-size and a large capacity in order to compress gas having a low pressure and a high flow rate in the case of using a low calorie gas (gas having a low calorific value) such as mine methane gas as the fuel gas. Meanwhile, the fuel gas supply system in which fuel gas before being compressed by the gas compressor is fed into the air passage upstream of the turbocharger, possibly causes a risk of detonating the fuel gas at the outlet of the turbocharger since inflammable fuel gas is compressed up to a high temperature and a high pressure within the turbocharger.
Accordingly, Patent Document 2 (Japanese Patent Laid Open No. 2006-249954) discloses the configuration that the fuel gas in one of both systems is mixed with air at the inlet of the turbocharger in order to feed the thus obtained mixture into the turbocharger while the fuel gas in the other system is mixed with intake air in an intake passage for each of cylinders so as to feed the thus obtained mixture into the associated one of cylinders, and there are provided a turbocharger side fuel flow control valve for regulating the flow rate of gas in the turbocharger side gas supply passage, a cylinder side fuel flow control valve for regulating the flow rate of the gas in the cylinder side gas supply passage for each of the cylinders, and a gas flow controller for controlling the opening degree of the turbocharger side fuel flow control valve so as to regulate the quantity of fuel gas fed into the supercharge side gas supply passage in order to maintain the density of the fuel gas in the mixture fed into the turbocharger at a value which is not greater than an inflammable limit gas density. With this configuration, the detonation of fuel gas at the inlet of the turbocharger can be completely avoided, and as well, the drive power of the gas compressor for compressing fuel gas fed into the intake passage for each of the cylinders can be reduced even in the case of using a low calorie gas (gas having a low calorific value), thereby it is possible to reduce the size and the capacity of the gas compressor.
As stated above, in view of the configuration disclosed in the Patent Document 2, a sufficient supply quantity of fuel gas can be ensured even with a low calorie gas, and as well the size and the capacity of the gas compressor for compressing fuel gas can be reduced. However, there has been desired a control method in which the quantity of fuel gas to be mixed with air at the inlet of the turbocharger can be appropriately controlled with a simple configuration, and which can be applied to the system that the calorie of the fuel gas varies.